The present invention relates to an image reading device for use in a digital copier, facsimile transceiver or similar image recording equipment and capable of changing a magnification change ratio.
Magnification changing methods heretofore proposed in the past for use with an image reading device may generally be classified into an optical method which changes magnification mechanically by operating optics as in analog image processing, and an electrical method which changes it electrically by processing signals as in digital image processing. It has been customary with the optical method to change magnification in the main scanning direction by changing the length of the optical path of optics and change it in the subscanning direction by changing the speed of a carriage. This kind of method, however, is not applicable to, for example, optics of the type using a cohesion type image sensor. Even in optics using a reduction type image sensor, such a method has critical problems since magnification does not change by noticeable degrees despite the substantial displacements of the mirrors and other optical elements. Specifically, the available range of magnification change ratios is limited, it is difficult to enhance accurate movement of the mirrors and other components, and the position cannot be adjusted with ease. In light of this, a current trend in the imaging art is toward the replacement of the optical method with the electrical method which estimates magnification-changed image data by calculation from image data read from a document. For example, in digital processing of image data, magnification is changed by thinning a pixel signal representative of binary data or by using an interpolation function, i.e., a table for interpolating a pixel signal.
Typical of magnification changing methods based on the conventional magnification changing algorithm are as follows:
(1) nearest pixel substitution method; PA1 (2) interpixel distance linear allocation method; and PA1 (3) cubic function convolution method.
Besides the three methods mentioned above, an interpixel distance inversely proportional method and a nearby pixel area allocation method are known in the art which are analogous to the method (2). With any of the three methods, it is necessary to recognize the position of a new sampling point and determine pixel data of old sampling points surrounding the new sampling point as well as the distances thereof to the new sampling point.
The above-stated methods are relatively old practices and have been commonly practiced in the computer image processing art. Although such methods are practicable relatively easily if image data are first stored in a memory and then subjected to magnification change as in computer image processing, various limitations are not avoidable in the case where such processing is executed by hardware without using a memory.
A prerequisite with a digital copier, facsimile transceiver or similar image recording equipment is that, to execute magnification change processing at the time of image reading, image data read by raster scanning be written in by the same mode of raster scanning. Another prerequisite is that a data clock be maintained constant with no regard to the magnification change processing. Specifically, data having undergone magnification change processing has to be outputted in the same format and at the same speed as data having undergone optical magnification change, i.e. in an output mode particular to real-time processing. The situation is somewhat different when magnification change processing is considered in relation to the entire system of equipment such as a digital copier or facsimile machine. For example, if the printing speed of a printer can be changed at the time of recording, the data clock at the time of recording can be changed also. Further, if an image reading device has a large capacity memory and is constructed as a system which stores image data in the memory and then prints them out or transmits them, it is not always necessary for the image data be outputted in the output mode particular to real-time processing. However, the previously mentioned prerequisites have to be met when the image reading device or the magnification change processing section of the image recording equipment is constructed as an independent unit.
On the other hand, even the electrical magnification ratio changing method has a problem that image data is sometimes distorted to lower the accuracy of magnification ratio change, if the method is of the kind changing the magnification by thinning or inserting an image signal as stated earlier. The problem with the conventional interpolation function scheme is that it cannot accommodate optional magnification change ratios, as distinguished from several fixed magnification change ratios, since the table lacks conversion data. Another problem with such a scheme is that even though a table loaded with data on a 1% basis may be used, the magnification change ratio in the same scanning is fixed and cannot be changed and, therefore, the freedom regarding the magnification change is limited.